Wire bonding capillary

ABSTRACT

A wire bonding capillary ( 1 ), which is able to decrease wire extraction resistance and suitable for a long and low loop wire bonding process, comprises a through bore ( 5 ) for feeding a bonding wire, a funnel-shaped bore ( 3 ) for guiding the wire to the through bore, and a tapered portion ( 11 ) formed at an exit of the through bore. The through bore length is between 0.2 and 1.5 times the through bore diameter. The tapered portion is tapered in two steps.

TECHNICAL FIELD

The present invention relates to a capillary for bonding wires between asemiconductor chip and a leadframe, and the like. Particularly, itrelates to an improved wire bonding capillary, which is capable ofdecreasing wire slide resistance (extraction resistance) and suitablyapplied for a long and low loop wire bonding process.

BACKGROUND ART

A wire bonding capillary is a tube made of ceramics, which is mounted toa tip of a wire-feeding portion of a wire bonding apparatus. In general,each wire bonding capillary has a through bore for feeding a bondingwire, a funnel-shaped bore for guiding the wire to the through bore, anda chamfer formed at an exit of the through bore. Here, the through boreis formed at the smallest diametered portion in each wire passage of acapillary. The chamfer is generally formed to taper having an openingangle of between 45° and 60° ( shown θ1, θ2 in FIG. 1). Further, thecorner between the chamfer and the through bore may be grounded to beround.

An embodiment of Japanese Unexamined Patent Publication No. Shou59(1984)-191338 discloses the following wire bonding capillary. Thecapillary, according to the embodiment, has a through bore whosediameter (corresponding to d1 in FIG. 1) is 43 μm and a chamfer whoseopening angles( corresponding to θ1, θ2 in FIG. 1) are 45° and 15°.

In Japanese Examined Utility Model Publication No. Hei 1(1989)-42349,the following wire bonding capillary is disclosed as a preferable one;he through bore diameter (corresponding to d1 in FIG. 1) is the wirediameter plus 8-12 μm; each of the opening angles of the chamfers(corresponding to θ1, θ2 in FIG. 1) is 40-50° and 10-20°, respectively;and the chamfer length ( corresponding to h1 in FIG. 1) is the wirediameter×0.7-1.5.

However, none of the two prior arts discloses the through bore lengthand a technology trend for long and low loop wire bonding processes, asdescribed below, and its associated problems.

Recently, in VLSI (Very Large Scale Integration) and the like,miniaturization of chips and multiplication of pins for high integrationpurposes have been advanced further, so that a need for long and lowloop wire bonding processes becomes keen.

When a conventional wire bonding tool is used for carrying out a longand low loop wire bonding process, it is necessary to feed, from thethrough bore, the wire with a twice length longer than usual.Accordingly, the wire tends to twine and thus to yield a dispersion ofthe loop height due to sagging and excessive tautening. The sagging ofthe loop causes the wires to contact each other to lead to a shortcircuit failure. And also, the excessive tautening of the loop appliesexcess stress to a neck portion of a bonded ball to lead to adisconnection failure. Both failures are fatal defect on the wirebonding process and have to be completely dismissed.

In view of the above problems, the object of the present invention is toprovide a wire bonding capillary, which is suitable for a long and lowloop wire bonding process and is able to lower wire slide resistance.

DISCLOSURE OF THE INVENTION

In order to solve the above-mentioned problems, a wire bonding capillaryaccording to the present invention has a through bore for feeding abonding wire, a funnel-shaped bore for guiding the wire to the throughbore, and a chamfer formed at an exit of the through bore, wherein thethrough bore is formed at the smallest diametered portion in each wirepassage of a capillary and the bore has a length less than 1.5 times thethrough bore diameter.

As a result of experiment as mentioned below, it is fundamentally proventhat the smaller the slide resistance of a wire, the shorter the lengthof the through bore. The through bore diameter, for the requirement ofthe positioning accuracy of the wire, is determined by adding someminute clearance to the through bore diameter, and the wire has to bedrawn through a narrow place so that the slide resistance is high.Accordingly, it is desirable that the slide resistance will be loweredby making the through bore length as short as possible. From this viewpoint, the through bore length is preferably less than 0.5 times thethrough bore diameter.

In the wire bonding capillary according to the present invention, it ispreferable that a chamfer is so formed to widen an opening angle thereoftoward an exit of the through bore. When a long and low loop wire isarranged, an angle between the axis of the capillary and the wire tendsto get bigger so that the wire will sharply bent at the tip of thecapillary to produce a large resistance against pulling out the wirehere.

Therefore, at the tip of the capillary, the chamfer is formed to widenthe opening angle thereof toward the exit of the through bore, wherebythe wire can be smoothly introduced while being bent. Consequently, theresistance will be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a wire bonding capillary tip accordingto one embodiment of the present invention.

FIG. 2 shows a schematic view of an experiment for measurement of theextraction resistance of a wire, with the extraction angle of the wirebeing changed, in various wire bonding capillaries.

FIG. 3 shows a graph of a result of an experiment for measurement of theextraction resistance of the wire, with the bore length ratio to thethrough bore of the capillary and the extraction angle of the wire beingchanged. The abscissa shows the extraction angle and the ordinate showsthe extraction resistance.

FIG. 4 shows a graph of a result of an experiment for measurement of theextraction resistance of the wire, with a shape of the chamfer and anextraction angle of the wire being changed. The abscissa shows theextraction angle and the ordinate shows the extraction resistance.

The reference numerals in the drawings are described as follows:

1 a wire bonding capillary

3 a funnel-shaped bore

5 a wire through bore

7 a first chamfer

9 a second chamfer

11 a tapered portion

15 outer surface

17 a wire

THE PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

Referring to the attached drawings, the detailed embodiments of thepresent invention will be set forth.

FIG. 1 shows a sectional view of a wire bonding capillary tip accordingto one embodiment of the present invention.

A capillary 1 is a tube made of a ceramic sintered body. An outersurface 15 of the capillary 1 has a conic surface, tapered thinnertoward the tip( toward a lower direction in FIG. 1).

The capillary 1 has a funnel-shaped bore 3, a wire through bore 5 and atapered portion 11, from the top to the bottom of FIG. 1, respectivelyat the inside thereof.

The funnel-shaped bore 3 is a bore that leads a wire 17 to the throughbore 5. The through bore 5 is a straight and narrow bore as to locatethe wire 17 to a certain position.

A chamfer 7 is a portion at which the wire, fed from the capillary 1 toa slantly downward direction, bends. In the capillary 1 of thisembodiment, the tapered portion 11 is so formed to be tapered in twosteps and has the first chamfer 7 at the through bore side and a secondchamfer 9 at the exit side. Of the opening angles of both chamfers (theangle between the axis of the capillary and each of the chamfers, θ1, θ2in FIG. 1), the angle at the exit side is larger. The numerical value ofthe angle is described later.

By this structure, it is accomplished that stress at the wire bentportion can be reduced, whereby the wire can be fed smoothly.

Next, the experimental examples that were carried out in order tooptimize the angle and size of each portion will be explained.

FIG. 2 shows a schematic view of an experiment for measurement of theextraction resistance of a wire, the extraction angle of the wire beingchanged, in various wire bonding capillaries.

The capillary 1 is set with its tip upward, and the wire 17 is insertedin the capillary. The wire 17 with 30 μm diameter is used.

At the bottom end of the wire 17, a 0.6 g weight 31 is suspended. Thewire 17 is slantly fed from the tip of the capillary and coupled to atension gauge 33 located at the tip end of the wire 17. In this state,an extraction resistance force is measured while the wire 17 isextracted at a constant speed( 0.3 mm/sec). Here, the experiment iscarried out while the angle (the extraction angle α) between the axis ofthe capillary and the extracting direction of the wire 17 is changed to30°, 60°, 75° and 90°.

FIG. 3 shows a graph of a result of an experiment for measurement of theextraction resistance of a wire, with a value, (the bore length ratio tothe through bore of the capillary), obtained by dividing the throughbore length by the through bore diameter and an extraction angle of thewire being changed. The abscissa shows the extraction angle and theordinate shows the extraction resistance. The bore length ratio is shownin various symbols in FIG. 3.

The chamfer of the capillary in this experiment has the opening angle θ1of 45°, and the corner between the chamfer and the through bore isgrounded.

Understood from FIG. 3, the extraction resistance suddenly rises whenthe extraction angle exceeds 60°. FIG. 3 clearly shows that theoperation of long and low loop wire bonding causes the extraction angleto become large and then resistance of the capillary increases, so thatthe smoothness of feeding of the capillary is liable to be lost. As forthe bore length ratio to the through bore, the extraction resistancewill be lower when the ratio is lower. That is, because the extractionresistance at a narrow through bore is large, when the length of thethrough bore is shortened, the extraction resistance decreases.According to this Figure, the bore length ratio to the through bore ispreferably not bigger than 1.5 at which the extraction angle is 75° andthe extraction resistance is less than 4.0 g. The ratio is morepreferably not bigger than 1.0 at which the extraction angle is 75° andthe extraction resistance is less than 3.6 g. Further, the ratio is morepreferably not bigger than 0.5 at which the extraction angle is 75° andthe extraction resistance is less than 3.0 g. The lower limit of thebore length ratio to the through bore is preferably not smaller than 0.2in view for preventing the eccentricity of a bonded ball.

FIG. 4 is a graph showing a result of the extraction resistancemeasurement of a wire, while the shape of the chamfer and the extractionangle of the wire are changed. The abscissa shows the extraction angleand the ordinate shows the extraction resistance. The shapes of thechamfer are shown in various symbols in FIG. 4.

The capillary of the embodiment has two tapered portions( 15° and 45°)and the bore length ratio is 0.7. The capillary of the comparativeexample 1 has one tapered portion (45°) and the bore length ratio is3.0. The capillary of the comparative example 2 has one tapered portion(45°) and the bore length ratio is 2.0.

As shown FIG. 4, in the embodiment, the extraction resistance issubstantially low in comparison with the comparative examples 1 and 2.And, even when the extraction angle becomes 75° or 90°, the extractionresistance dose not greatly increase. On the other hand, in thecomparative examples, the extraction resistance is not only large as awhole but also the extraction resistance greatly increases as theextraction angle increases. By this, it is understood that the taperedportion, from which the wire is fed, is formed to widen toward the exitso that the extraction resistance can decrease.

The following results are obtained from the experiments conducted.

θ1 is 40-60°, preferably about 45°;

θ2 is 10-25°, preferably about 15°;

the through bore diameter d1 is the wire diameter d plus (6-15) μm,preferably plus 12 μm;

the through bore length h2 is the through bore diameter d1×(0.2-1.5),preferably×0.7.

When the edges of the lower chamfer 9, the upper chamfer 7 and theboundary part of the through bore are slightly rounded, the best resultwill be obtained.

Next, materials and manufacturing methods, according to this embodiment,of the wire bonding capillary will be explained.

Material examples are alumina based ceramics, ruby and silicon nitridebased ceramics.

Forming methods include a press molding, an isostatic molding and aninjection molding method.

Polishing is an example of a through bore machining method.

Tapered portion machining methods include grinding and polishing.

What the wire bonding capillary according to the present embodiment isemployed, in a case where the wire is fed to a slantly downwarddirection, the capillary acts as a guide so that the feeding of the wirecan be smoothly carried out and wire bonding failures associated with adispersion of the loop height can be diminished.

The present invention, as described above, provides a wire bondingcapillary which is able to decrease wire extraction resistance andsuitable for a long and low loop wire bonding process. Thus, the wirebonding capillary according to the present invention can be adapted forfurther high integration of the VLSI.

Various modifications and alteration can be made within the scope of theappended claims. Features of one embodiment can be combined with otherembodiments, consistent with proper operation. While the principles ofthe invention have been described above in connection with specificapparatus, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

We claim:
 1. A wire bonding capillary, comprising: a through bore forfeeding a bonding wire; a funnel-shaped bore for guiding the wire tosaid through bore; and a chamfer formed at an exit of said through bore,wherein said through bore has a smallest diametered portion with alength less than 1.5 times its diameter.
 2. The wire bonding capillaryaccording to claim 1, wherein said length is more than 0.2 times saiddiameter.
 3. The wire bonding capillary according to claim 1, whereinsaid chamfer is so formed to widen an opening angle thereof toward saidexit of said through bore.
 4. The wire bonding capillary according toclaim 1, wherein said chamfer is tapered in two steps.
 5. The wirebonding capillary according to claim 1, wherein: said chamfer is taperedin two steps; said first step has an opening angle of about between 10and 25°; and said second step has an opening angle of about between 40and 60°.
 6. The wire bonding capillary according to claim 1, wherein:said length is more than 0.2-1.5 times said diameter; said chamfer istapered in two steps; said first step has an opening angle of aboutbetween 10 and 25°; and said second step has an opening angle of aboutbetween 40 and 60°.
 7. The wire bonding capillary according to claim 1,wherein: said diameter is the wire diameter plus 6-15 μm.
 8. A wirebonding capillary, comprising: a through bore for feeding a bondingwire; a funnel-shaped bore for guiding the wire to said through bore;and a chamfer formed at an exit of said through bore, wherein saidthrough bore has a smallest diametered portion with a length less than1.0 times its diameter.
 9. The wire bonding capillary according to claim8, wherein said length is more than 0.2 times said diameter.
 10. A wirebonding capillary according to claim 8, wherein said chamfer is soformed to widen an opening angle thereof toward said exit of saidthrough bore.
 11. The wire bonding capillary according to claim 8,wherein said chamfer is tapered in two steps.
 12. The wire bondingcapillary according to claim 8, wherein: said chamfer is tapered in twosteps; said first step has an opening angle of about between 10 and 25°;and said second step has an opening angle of about between 40 and 60°.13. The wire bonding capillary according to claim 8, wherein saiddiameter is the wire diameter plus 6-15 μm.